Many portable electronic devices such as handheld wireless communications devices (mobile phones or smart phones) and notebook or laptop personal computers have built-in, switching power supply circuits that are powered by an external power source. For instance, when an AC power adapter having a regulated output is plugged into a smart phone, a switching power supply circuit in the smart phone draws current from the plugged-in power adapter, and produces the needed voltage for charging the device's battery and for operating the rest of the components of the device. The way in which the switching power supply circuit draws current, however, creates a negative impedance load. As a result, the external power source sometimes behaves abnormally, e.g. its normally stable dc output drops out of regulation, exhibits excessive undershoot and overshoot, and may even oscillate.